1. Technical Field
The present invention relates to an optical device, a transfer mold, and an image pickup apparatus, and more particularly, to a technology of an optical device used for an image pickup apparatus.
2. Related Art
Digital video cameras that convert an incident light into an electrical signal by the use of an image pickup device and that store an image of a subject as digital data have been spread as an electronic apparatus using an image pickup device. The image pickup devices used in the digital video cameras have light-receiving elements regularly arranged to correspond to pixels. A moire may occur in an image recorded in the image pickup device due to the overlapping of a pixel structure of the image pickup device with a shape of a subject. The moire may occur at the time of reproducing an image due to the overlapping of a periodic structure of a reproducing apparatus for reproducing an image with the image. The moire allows colors or shapes not included in the original subject to appear, thereby causing a decrease in image quality. As a countermeasure against the moire, a technology using a low pass filter was known in the past. For example by using a birefringent plate such as a quartz plate splitting an incident light as the low pass filter, the periodicity of an image is reduced. In order to effectively reduce the periodicity of an image, it is preferable that the number of split lights obtained by splitting an incident light increases. The number of birefringent plates to be prepared for an incident light is “(the number of split lights)×2−1) and the birefringent plate corresponding to the distance by which the light is shifted need be prepared. Accordingly, since the more birefringent plates are required with an increase in the number of split lights, it is difficult to decrease in size of the image pickup apparatus, thereby causing an increase in cost. On the contrary, a technology using a diffraction grating as a low pass filter is disclosed in JP-A-4-372925. For example, a technology using an optical device diffracting light as a low pass filter is disclosed in JP-A-4-67010.
The diffraction grating disclosed in JP-A-4-372925 generates a second-order diffracted light or a third-order diffracted light which is an unnecessary high-order diffracted light in addition to a desired first-order diffracted light. In the optical device with a regular pattern like the configuration disclosed in JP-A-4-67010, an undesired diffracted light may be generated due to an effect of the low pass filter depending on the pitches of the optical device. The undesired diffracted light causes a blur of an image or a decrease in contrast.